This invention pertains to heat exchange structures, particularly such structures made of refractory materials.
Such heat exchangers are used in blast furnace stoves, glass tank checkers, and other applications. The present invention will be described with respect to its use in glass tank checkers since it has found particular application there. However, it will be apparent to those skilled in the art that it has wider application.
It is common to make glass tank checkers of standard refractory brick, that is to say, rectangular shaped 9.times.41/2.times.21/2 inches (23.times.11.5.times.6.4 cm) in size. However, the checkers in glass tank regenerators can be constructed in many different ways from this single standard shape, and the design of a checker or heat exchanger requires careful balance of several different factors.
One factor is the cross-sectional area of the flues or vertical channels through the checker. (It is common to construct the heat exchangers associated with glass tanks so that the gases pass vertically through the checker. However, it will be evident to those skilled in the art that the principles of the present invention are equally applicable to heat exchangers wherein the gases flow horizontally.) For purposes of maximum heat exchange, narrow flues are desirable. However, narrow flues lead to large pressure drops across the checker in order to move the requisite amount of gas through. Also, narrow flues tend to be more easily clogged, for example by material carried out of the glass tank by the hot gases flowing from the tank to the heat exchanger.
Another factor is the thickness of the refractory walls between the flues. The thinner these walls, the more quickly heat will penetrate to the center of the wall, better utilizing the mass of the brick. On the other hand, more heat can be stored in a thicker refractory wall. Perhaps more importantly, if the wall is too thin, the checker structure can be mechanically unstable and may collapse. (It is customary in these structures to place the standard refractory shape lengthwise on its side so that the refractory wall between flues is 2.5 inches (6.4 cm) thick and the height of a single layer or level of brick in the checker is 4.5 inches (11.5 cm). While it would be quite easy to make thinner brick, for example brick 1.5 inches (3.8 cm) thick, it appears that the 2.5 inch (6.4 cm) dimension is the minimum thickness which will yield a satisfactorily stable checker structure using standard brick shapes.)
Another factor to be considered is the amount of solid refractory in a unit volume of checker structure. The higher this figure, the greater is the amount of heat which can be stored in the unit volume. This factor is interrelated with flue size and wall thickness. For example, a large amount of solid material per unit volume of checker implies less flue volume, with its attendant disadvantages.
Still another factor is the amount of heat transfer surface available per unit mass of solid refractory. It is desirable that this factor be large so that there can be more rapid heat transfer from the gases to the refractory or vice versa. (It will be understood that, in a glass tank checker, the checker brick are first heated by hot combustion gases flowing from the glass melting furnace down through the checker to the exhaust stack and that then the flow of gas through the checker is reversed, cool incoming air for combustion flowing up through the checker to be preheated before it enters the glass tank to support combustion of the fuel therein.)
While a higher ratio of surface area to volume of solid material is desirable for heat transfer, it implies larger flue volume and relatively less solid material per unit volume of checker, together with the concomitant disadvantages thereof.
Thus it can be seen that the designer of a heat exchanger, for example a glass tank checker, is faced with a multitude of choices and is seeking to construct the most efficient heat exchanger with the materials available. A brief examination of some prior art checker structures will indicate how these factors have been handled in practice.
FIG. 1 shows a well-known type of checker structure known as "basketweave". As can be seen from the figure, for a given size of brick, this structure results in relatively small flues which are continuous from top to bottom of the structure and which have no interconnection with any adjacent flues. While this structure is very stable, it has one big disadvantage: if a blockage occurs somewhere within the flue, that flue is rendered totally inoperative and thus reduces the capacity of the checker, as well as increasing the pressure drop necessary to force a given amount of gas through the checker.
FIG. 2 shows a closely related, and commonly used, checker structure known as "open basketweave". Here, as can be seen from the figure, the individual brick are laid in the same arrangement as in the basketweave structure, but are spaced horizontally from each other. The result is continuous vertical flues, each of which is connected at each level in the structure to the four contiguously adjacent flues. One result of this structure is that the flues are slightly larger than the basketweave flues and that, concomitantly, there is less solid refractory material per unit volume of the checker. However, perhaps the most important feature of this open structure is that if there is blockage at some point in the flue, the gases passing down that flue can flow outwardly to adjacent flues and back into the blocked flue below (or above, depending on the direction of gas flow) the blockage. Thus, a blockage does not so significantly affect the capacity of the checker. Another advantage of this open structure is the introduction of turbulence into the gases flowing through the checker, with resulting better heat exchange between the gas and the solid.
FIG. 3 shows a third type of prior art checker structure, one known as "pigeonhole" sometimes called open-flue construction. As can be seen from the figure, this leads to the largest size flues of the three structures discussed, but at the same time has the least amount of solid refractory per unit volume of checker. On the other hand, the very open structure of the pigeonhole construction minimizes problems with clogging and generally lowers the pressure drop required across the structure (with the consequent savings in fans and motors needed to move the hot gases).
In addition to the structures discussed using standard refractory shapes, there are many special shapes which have been suggested exclusively for use as heat exchanger shapes. See, for example, U.S. Pat. No. 2,451,392, U.S. Pat. No. 3,456,937, and U.S. Pat. No. 4,282,923. Fundamentally, these have been attempts to achieve maximum thermal efficiency in a given type of heat exchanger for a given application. While it is not practical to discuss each of these prior art shapes in detail, it can be said that none discloses the heat exchanger structure of the present invention.
The present invention is directed to a refractory shape and heat exchange structure constructed therefrom which increases the thermal efficiency of a heat exchanger as compared with prior art structures, particularly with respect to stability of the structure, the thinness of the walls between flues, and the introduction of turbulence into the gases flowing through the structure.